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Rewriting plugins: request for feedback
by Sam Derbyshire 23 Jul '21

23 Jul '21

Hi everyone, Recent refactors to the constraint solver have complicated the story for authors of type-checking plugins. Following a suggestion by Christiaan Baaij, and aided by Richard Eisenberg, I'm working on adding the ability for type-checking plugins to rewrite type family applications directly (hooking straight into GHC's type-family rewriting code). The goal is to ease migration for the plugin authors, hopefully simplifying the common operation of rewriting type family applications. Let me now describe the current API for this new functionality, for which I would like feedback. -------------------------------------------------------------------------- A new field is added to the TcPlugin data type, namely tcPluginRewrite :: s -> UniqFM TyCon TcPluginRewriter Here s is the existential plugin state that users can choose as they wish for their own plugin. Plugins thus register which type family TyCons they are interested in rewriting, like so: myTcPluginRewrite :: MyPluginState -> UniqFM TyCon TcPluginRewriter myTcPluginRewrite s@(MyPluginState {myFam1TyCon, myFam2TyCon}) = listToUFM [ (myFam1TyCon, myFam1Rewriter s), (myFam2TyCon, myFam2Rewriter s) ] For each type family TyCon, the plugin should provide a rewriting function (corresponding to myFam1Rewriter and myFam2Rewriter above), of type type TcPluginRewriter = [Ct] -> [TcType] -> TcPluginM TcPluginRewriteResult This means that a rewriting function is supplied with the current Given constraints and the arguments of the (guaranteed to be saturated) type-family application, and should return a result of type: data TcPluginRewriteResult where TcPluginRewriteError :: (Diagnostic a, Typeable a) => a -> TcPluginRewriteResult TcPluginNoRewrite :: TcPluginRewriteResult TcPluginRewriteTo :: { rewriteTo :: TcType , rewriteEvidence :: TcCoercion } -> TcPluginRewriteResult That is, the plugin can specify what the type-family application should be rewritten to, throw an error, or do nothing. -------------------------------------------------------------------------- I'm hoping that the components of existing type-checking plugins concerned with type-family applications can instead use this mechanism and achieve more robust results by doing so, with less effort. I would like to hear from plugin authors whether this API suits their needs. If you know of someone not on this list that might be interested, please invite them to join the conversation. The current state of this patch can be found in the GHC MR 5909: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5909 Thanks, Sam

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Haskell program introspection tooling development.
by Csaba Hruska 21 Jul '21

21 Jul '21

Hello, I'm using the external STG interpreter to introspect the runtime behaviour of Haskell programs. Lately I've added an initial call-graph construction feature that I plan to refine further. https://twitter.com/csaba_hruska/status/1417486380536582151 Is there anyone who has dynamic analysis related research ambitions and wants to study Haskell program runtime behaviour in detail? If so then it would be great to talk. Cheers, Csaba

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FYI: Darwin CI currently broken for forks
by Matthew Pickering 20 Jul '21

20 Jul '21

Hi all, There is a configuration issue with the darwin builders which has meant that for the last 6 days CI has been broken if you have pushed from a fork because the majority of darwin builders are only configured to work with branches pushed to the main project. These failures manifest as timeout errors (https://gitlab.haskell.org/blamario/ghc/-/jobs/733244) Hopefully this can be resolved in the coming days. Cheers, Matt

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mkIfaceTc panics with "lookupVers1"
by Erdi, Gergo 13 Jul '21

13 Jul '21

PUBLIC Hi, I'm trying to use `mkIfaceTc` to make a ModIface from the results of typechecking. Everything goes well until it gets to `makeFullIface`, where it fails to find some imported fingerprints: hello: hello: panic! (the 'impossible' happened) (GHC version 9.0.1: lookupVers1 MyPrim Foo Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/GHC/Utils/Outputable.hs:1230:37 in ghc-lib-9.0.1.20210623-3xx7a2u7IkN9vKAnkscROb:GHC.Utils.Outputable pprPanic, called at compiler/GHC/Iface/Recomp.hs:1455:19 in ghc-lib-9.0.1.20210623-3xx7a2u7IkN9vKAnkscROb:GHC.Iface.Recomp I am using the GHC API to load my modules differently than what GHC itself does; I am attaching the full code, but perhaps to note is that I am creating ModSummarys one by one and then typechecking and adding them to the moduleNameProviderMap in dependency order. Also, I am using `downsweep` directly instead of `depanal`, because the latter's `flushFinderCaches` was breaking my inter-unit imports. Because the attached code is a minimized version of something larger, some parts of it may seem to be doing things in a roundabout way but that's because in the real program all those degrees of freedom are used. In case that matters, the only packages in my package DB are rts, ghc-prim-0.7.0 and ghc-bignum-1.0. So my question is basically, what am I doing wrong? Why is fingerprinting failing on `Top.hs`'s import of `MyPrim.hs`, and how do I fix that, while still keeping this total control over when and how modules are typechecked and loaded? Thanks, Gergo This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at https: //www.sc.com/en/our-locations Where you have a Financial Markets relationship with Standard Chartered PLC, Standard Chartered Bank and their subsidiaries (the "Group"), information on the regulatory standards we adhere to and how it may affect you can be found in our Regulatory Compliance Statement at https: //www.sc.com/rcs/ and Regulatory Compliance Disclosures at http: //www.sc.com/rcs/fm Insofar as this communication is not sent by the Global Research team and contains any market commentary, the market commentary has been prepared by the sales and/or trading desk of Standard Chartered Bank or its affiliate. It is not and does not constitute research material, independent research, recommendation or financial advice. Any market commentary is for information purpose only and shall not be relied on for any other purpose and is subject to the relevant disclaimers available at https: //www.sc.com/en/regulatory-disclosures/#market-disclaimer. Insofar as this communication is sent by the Global Research team and contains any research materials prepared by members of the team, the research material is for information purpose only and shall not be relied on for any other purpose, and is subject to the relevant disclaimers available at https: //research.sc.com/research/api/application/static/terms-and-conditions. Insofar as this e-mail contains the term sheet for a proposed transaction, by responding affirmatively to this e-mail, you agree that you have understood the terms and conditions in the attached term sheet and evaluated the merits and risks of the transaction. We may at times also request you to sign the term sheet to acknowledge the same. Please visit https: //www.sc.com/en/regulatory-disclosures/dodd-frank/ for important information with respect to derivative products.

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Can NamedFieldPuns be added to `GHC.LanguageExtensions.Types.Extension`?
by Alfredo Di Napoli 13 Jul '21

13 Jul '21

Dear all, As some of you might know, for the past few months I have been working on changing GHC's diagnostic messages from plain SDocs to richer Haskell types. As part of this work, I have added a mechanism to embed hints into diagnostics, defined in `GHC.Types.Hint` in `HEAD`. One of the main workhorse of this `GhcHint` type is the `SuggestExtension LangExt.Extension` constructor, which embeds the extension to enable to use a particular feature. The `LangExt.Extension` type comes from `GHC.LanguageExtensions.Types`, and up until now there has always been a 1:1 mapping between the language pragma for the extension and the type itself. Today I was working on turning this error into a proper Haskell type: badPun :: Located RdrName -> TcRnMessage badPun fld = TcRnUnknownMessage $ mkPlainError noHints $ vcat [text "Illegal use of punning for field" <+> quotes (ppr fld), text "Use NamedFieldPuns to permit this"] I was ready to yield a `SuggestExtension LangExt.NamedFieldPuns` when I discovered that there is no `NamedFieldPuns` constructor. Rather, there is a `RecordPuns` , which refer to a deprecated flag, and we simply map `NamedFieldPuns` back to it in `GHC.Driver.Session`: ... depFlagSpec' "RecordPuns" LangExt.RecordPuns (deprecatedForExtension "NamedFieldPuns"), ... flagSpec "NamedFieldPuns" LangExt.RecordPuns, ... This is problematic for the `GhcHint` type, because now if I was to yield `SuggestExtension LangExt.RecordPuns` to the user, I could still pretty-print the suggestion to turn `RecordPuns` into `NamedFieldPuns`, but this means that IDEs or third-party library would have access to the "raw" Haskell datatype, and at that point they will be stuck with a suggestion to enable a deprecated extension! (or best case scenario they will have to transform the suggestion into something more sensible, which partially defeats the point of this refactoring work I have been doing). I am not sure this behaviour is unique for just `NamedFieldPuns`, but my question is: 1. What prevents us from adding `NamedFieldPuns` as a proper constructor for the `Extension` type and in principle remove `RecordPuns`? Backward compatibility I assume? Many thanks, Alfredo

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Re: Trying to speedup GHC compile times...Help!
by Jeffrey Young 10 Jul '21

10 Jul '21

Thanks for pointing out the linking issue. Looks like the table of content references get clobbered by gitlab. Unfortunately, pandoc doesn't generate the table of contents, and the gitlab org parser ignores #+OPTION: toc:3. So I've opted to just remove the table of contents entirely. - Jeff On 7/9/21 5:00 AM, ghc-devs-request(a)haskell.org wrote: > Send ghc-devs mailing list submissions to > ghc-devs(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > or, via email, send a message with subject or body 'help' to > ghc-devs-request(a)haskell.org > > You can reach the person managing the list at > ghc-devs-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of ghc-devs digest..." > > > Today's Topics: > > 1. Re: Trying to speedup GHC compile times...Help! (Jeffrey Young) > 2. Re: Trying to speedup GHC compile times...Help! (Jaro Reinders) > 3. Re: Trying to speedup GHC compile times...Help! (Brandon Allbery) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Thu, 8 Jul 2021 13:04:48 -0700 > From: Jeffrey Young <jeff.young(a)tweag.io> > To: ghc-devs(a)haskell.org > Subject: Re: Trying to speedup GHC compile times...Help! > Message-ID: <931b340b-d9ec-5702-823c-92b56f01f6a2(a)tweag.io> > Content-Type: text/plain; charset="utf-8"; Format="flowed" > > Hi everyone, > > > I've created a wiki page here > <https://gitlab.haskell.org/ghc/ghc/-/wikis/Adventures-in-GHC-compile-times> > that collected your suggestions and tracks my work. I'll be regularly > updating it. Any comments welcome. > > > - Jeff > >

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Re: Trying to speedup GHC compile times...Help!
by Jeffrey Young 09 Jul '21

09 Jul '21

Hi everyone, I've created a wiki page here <https://gitlab.haskell.org/ghc/ghc/-/wikis/Adventures-in-GHC-compile-times> that collected your suggestions and tracks my work. I'll be regularly updating it. Any comments welcome. - Jeff

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Why base library changes are only discussed on GHC issue tracker and not on the libraries@ list?
by Oleg Grenrus 08 Jul '21

08 Jul '21

For example - https://gitlab.haskell.org/ghc/ghc/-/issues/20044 ByteArray migration from primitive to base - https://gitlab.haskell.org/ghc/ghc/-/issues/20027 Changing Show String behavior Why they are discussed "in private", I thought libraries@ list is where such changes should be discussed. - Oleg

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Marking ParsedModule fragments as non-user-originating
by Erdi, Gergo 06 Jul '21

06 Jul '21

PUBLIC Hi, I'd like to hijack some syntax (like string literals or list patterns) for my own use, and I thought a low-tech way of doing that is to transform the ParsedModule before typechecking. For example, if I have a function `uncons :: Array a -> Maybe (a, Array a)`, I can rewrite the pattern `[x1, x2, x3]` into the view pattern `(uncons -> Just (x1, (uncons -> Just (x2, (uncons -> Just (x3, (uncons -> Nothing)))))))` and let the normal GHC type checker take over from here. This is working for me so far, except for one problem: the typechecker now complains that the `ViewPatterns` language extension is not turned on. I would like to make the view patterns coming from my ParsedModule rewriter to be exempt from this check (but of course still require the `ViewPatterns` extension for user-originating code). Is there a way to do that? Or would I be better off checking for user-originating view patterns myself before the rewrite and then changing the `DynFlags` to always enable view patterns for typechecking? Thanks, Gergo This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at https: //www.sc.com/en/our-locations Where you have a Financial Markets relationship with Standard Chartered PLC, Standard Chartered Bank and their subsidiaries (the "Group"), information on the regulatory standards we adhere to and how it may affect you can be found in our Regulatory Compliance Statement at https: //www.sc.com/rcs/ and Regulatory Compliance Disclosures at http: //www.sc.com/rcs/fm Insofar as this communication is not sent by the Global Research team and contains any market commentary, the market commentary has been prepared by the sales and/or trading desk of Standard Chartered Bank or its affiliate. It is not and does not constitute research material, independent research, recommendation or financial advice. Any market commentary is for information purpose only and shall not be relied on for any other purpose and is subject to the relevant disclaimers available at https: //www.sc.com/en/regulatory-disclosures/#market-disclaimer. Insofar as this communication is sent by the Global Research team and contains any research materials prepared by members of the team, the research material is for information purpose only and shall not be relied on for any other purpose, and is subject to the relevant disclaimers available at https: //research.sc.com/research/api/application/static/terms-and-conditions. Insofar as this e-mail contains the term sheet for a proposed transaction, by responding affirmatively to this e-mail, you agree that you have understood the terms and conditions in the attached term sheet and evaluated the merits and risks of the transaction. We may at times also request you to sign the term sheet to acknowledge the same. Please visit https: //www.sc.com/en/regulatory-disclosures/dodd-frank/ for important information with respect to derivative products.

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Trying to speedup GHC compile times...Help!
by Young, Jeff 06 Jul '21

06 Jul '21

Hi ghc devs, I'm a long-time Haskeller but am just getting into GHC development. I started a 12 week internship at Tweag I/O under Richard Eisenberg this week with the singular goal to speedup GHC compile times. I'm specifically looking to contribute to ghc issues 18541 <https://gitlab.haskell.org/ghc/ghc/-/issues/18541> and 18535 <https://gitlab.haskell.org/ghc/ghc/-/issues/18535>. So I thought I would reach out to the community to get some direction on issues/features/problems to tackle in the pursuit of faster compilation times. This is a full time internship and so I think there is a real opportunity to nail down a deliverable for the community, but I want to get some guidance from the experts (you fine people!) before going down a rabbit hole. To be specific I'm looking for lingering items such as: 1. It would be great if we had <thing-here> but no one has time. 2. Primop foo is half complete but is the right type for <common-use-case-which-is-currently-just-a-list>. 3. Swap <some-type> to an array-like type *non-incrementally*, that is, establish a patch that rips out the previous type and replaces it with the array-like across the entire compiler, rather than module-by-module. Point 2 is a specific reference to MR 3571 <https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3571> but I'm unsure of the status and etiquette around MRs, and I'm unsure exactly how fulfilling the todos at the end of that MR would aid in faster compilation times (and if there is evidence to that effect somewhere). Thanks for the help! - Jeff

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